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The GOP's comments come as the White House readies a gun control executive action

Washington (CNN)Republican presidential contenders are threatening to undo any executive actions that President Barack Obama takes on gun control if they replace him in the Oval Office.

As the White House readies an executive push expected to close the so-called gun show loophole on background checks for firearms purchases, GOP 2016 candidates said this weekend that they're opposed to Obama's push, arguing he shouldn't circumvent Congress.

"There's an assault on the Second Amendment. You know Obama's going to do an executive order and really knock the hell out of it," Trump said.

"You know, the system's supposed to be you get the Democrats, you get the Republicans, and you make deals. He can't do that. He can't do that," he said. "So he's going to sign another executive order having to do with the Second Amendment, having to do with guns. I will veto. I will unsign that so fast."

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But he's not the only Republican 2016 candidate making such threats.

"All these executive orders he's gonna come out with tomorrow that are going to undermine our Second Amendment rights -- on my first day in office, they're gone," Florida Sen. Marco Rubio said at a campaign stop in New Hampshire on Sunday morning.

New Jersey Gov. Chris Christie said on "Fox News Sunday" that Obama "wants to act as if he is a king, as if he is a dictator."

"Fact is, if he wants to make changes to these laws, go to Congress and convince the Congress that they're necessary. But this is going to be another illegal executive action, which I'm sure will be rejected by the courts and when I become President will be stricken from executive action by executive action I'll take."

And former Florida Gov. Jeb Bush said on "Fox News Sunday" that Obama should take his case to Congress -- rather than imposing new measures himself.

"His first impulse always is to take rights away from law-abiding citizens, and it's wrong. And to use executive powers he doesn't have is a pattern that is quite dangerous," Bush said.

Carly Fiorina told CNN's Dana Bash on "State of the Union" that Obama "has been lawless in his use of executive orders.

"It is delusional, dangerous, not to mention unconstitutional, for Barack Obama and Hillary Clinton to continue talking about climate change and gun control in the wake of a Paris terrorist attack, in the wake of a San Bernardino terrorist attack, instead of how we can defeat ISIS," Fiorina said.

Still, Democrats have rallied to Obama's side.

Vermont Sen. Bernie Sanders said on "State of the Union" that Obama has little choice but to act alone to strengthen background checks to keep guns out of the hands of criminals and the mentally ill.

"I would prefer that we could have bipartisan support, but the truth is Republicans aren't interested in doing anything on gun safety," Sanders said.

Democratic front-runner Hillary Clinton said Sunday she's "especially concerned (about 2016) because I know what a Republican president would mean."

She argued a GOP president would repeal executive actions on Day 1, "including one that we expect (Obama) to make in the next weeks to try to do more to have background checks for more gun buyers by requiring more sellers to do them."